In order to produce an optical waveguide device using a substrate made of an electro-optic crystal, such as LiNbO3 (LN) or LiTaO2, a metal film, such as a Ti film, is formed on part of the substrate and the metal is thermally diffused, or such a film is patterned and then subjected to proton exchange with benzoic acid. Subsequently, an electrode is formed in the vicinity of the optical waveguide to complete the optical waveguide device.
For example, a Mach-Zehnder optical modulator is used which includes an incident side waveguide, a pair of waveguides and an emission side waveguide in a photoelectric optical substrate, and coplanar RF (radio frequency) and ground electrodes over the parallel waveguides (for example, Japanese Laid-open Patent Publication No. 2005-265959). The operating point voltage, at which the optical output is turned off, of the Mach-Zehnder optical modulator, is varied depending on environmental temperature and other factors.
Accordingly, the Mach-Zehnder optical modulator is provided with a bias electrode in addition to the RF electrode for applying radio frequency signals. Output light is monitored while a low-frequency bias voltage is applied to the bias electrode so as to keep the operating point voltage constant, and thus the bias voltage is controlled according to the intensity of the output light. A variety of methods, such as multilevel modulation and polarization multiplexing, have been applied for optical modulation through the years. Accordingly, there are increasing cases of using a plurality of optical modulators. When a plurality of optical modulators are used, the optical modulators may be integrated on a substrate to reduce the size of the optical device.